by Oren Dorell, USA TODAY

by Oren Dorell, USA TODAY

Iran has been slow to explain away evidence that it conducted experiments and developed plans that experts say are only good for building a nuclear bomb, and that the U.S. says must be explained if a deal on Iran's disputed nuclear program is to be reached by a July 20 deadline.

And now former Obama administration officials and nuclear experts who advise the administration disagree on how much Iran needs to come clean on its past nuclear activities.

A senior U.S. official complained to reporters as negotiators wrapped up three days of talks between world powers and Iran in Vienna on Friday of "a lack of urgency on the Iranian side," and said Iran needs to address "past and present concerns" of the United Nations nuclear watchdog, the IAEA.

The State Department's chief Iran negotiator, Wendy Sherman, told members of the Senate Committee on Foreign Relations in February that current talks depend on "resolution" of IAEA questions about "possible military dimensions" of Iran's nuclear program.

The International Atomic Energy Agency is seeking answers to questions compiled three years ago about how far Iran got on developing missile warheads, detonators that can spark a nuclear blast, whether its military is still developing nuclear facilities, like the uranium fuel processing plant it built under a mountain in Fordow, and whether such work continues.

Iranian answers are crucial to Western understanding of how close Iran is to a nuclear test or a deliverable bomb if it chose to renege on an agreement to curtail its nuclear activities, said David Albright, president of the Institute for Science and International Security, who has advised the Obama administration and testified before Congress on Iran's nuclear program.

"All of it is relevant to the understanding of whether Iran has capabilities we don't know about," Albright said. "If you want to have all this settled they have to stick to the timeline and accelerate it. If it goes on like it is now there's no hope to address any of the outstanding questions."

Demanding Iran provide a detailed comprehensive accounting, however, would be embarrassing to Iran's leadership, which has consistently denied that its program has anything but peaceful aims, and could interfere with the Obama administration's chief priority of preventing an Iranian nuclear weapon, said Jofi Joseph, former director for non-proliferation at President Obama's National Security Council.

Insisting on a full accounting "jeopardizes what's really important, which is making sure Iran's production of fissile material is severely constrained," Joseph says. The administration's first priority, he says, should be "to ensure that Iran, whatever it may have done in the past, is not proceeding today with weaponization activities in the future."

The Atomic Energy Organization of Iran said last month it is drafting a comprehensive accounting of Iran's nuclear activities, and it shared some information about suspected detonators May 4, but three diplomats told the Associated Press that Iran continues to refer to them as exploding bridge wires without explaining where or why they were used.

Iran denies it seeks a nuclear weapon and says evidence provided to the IAEA was provided by U.S. and Israeli intelligence agencies, includes electronic documents that had been manipulated, and were never provided to Iran to verify. Iranian President Hassan Rouhani said last week that Iran has a right to do nuclear research, and that efforts to stop it are akin to "scientific apartheid."

World powers are seeking a comprehensive agreement that would lift crippling sanctions on Iran's economy in return for it limiting its nuclear program to peaceful activities and resolving the IAEA's doubts about "possible military dimensions" of its nuclear program.

Olli Heinonen, a former deputy director of the IAEA, said the West might find a middle ground by not insisting on certain answers.

"You don't need a confession from them," Heinonen said. "Let's say they used high explosives in Parchin," a military facility where the IAEA suspects Iran conducted experiments on nuclear detonators. "You go there, see what they did and you agree to monitor it - they don't lose face and you get what you want."

At issue are hundreds of questions outlined by the IAEA in 2011 that have never been answered. The IAEA report was based on documents provided by member states, its own research of Iranian scholarly publications, and interviews with people who've worked with Iranian nuclear experts. It described evidence that Iran had worked on detonator designs that would fit in the warhead capsule of its Shahab 3 long-range missile, and that Iran experimented with such detonators and researched preparations for explosive tests in a mine shaft â?? all "highly relevant to a nuclear weapon program," according to the report.

â?¢ Iran has been provided with nuclear explosive design information by a member of a clandestine nuclear supply network.

â?¢ It had prepared to fabricate highly enriched uranium metal components for a nuclear explosive device, an important step in the development of a nuclear weapon.

â?¢ Iran acknowledged working on "exploding bridgewire detonators," concentric detonation devices that could also be used to detonate a nuclear weapon, but "has not explained to the agency its own need or application for such detonators," the report said.

â?¢ Iran tested whether such detonators would fire when controlled from a long distance away, while at the bottom of a deep shaft.

â?¢ Iran held large-scale experiment in 2003 with high explosives in a hemispherical shell "consistent with the dimensions for the new payload" chamber of the Shahab 3 missile's re-entry vehicle.

â?¢ A document in Farsi described "safety arrangements that would be necessary for conducting a nuclear test."

â?¢ Engineering studies of a new payload chamber for the Shahab 3 missile included the manufacture of components in workshops that Iran refused to allow IAEA inspectors to visit.

â?¢ Studies on another firing system would enable a payload to explode at high altitude or upon impact of a re-entry vehicle. The IAEA concluded that "any payload other than a nuclear weapons â?¦ could be ruled out." Iran agreed when confronted, according to the report, that if the information it was based on was true â?? which it denies â?? "It would constitute a program for the development of a nuclear weapon."